Onions need full sun and prefer a loose, rich, well-drained soil. They are very shallow rooted so need regular irrigation to avoid drying out, and benefit from careful mulching. They do not perform well if they are crowded or have to compete with weeds.
Growing from Seed
This is an economical way to grow onions and gives a chance for several different varieties to be compared. Onions may be sown directly into the ground early in the year for intermediate day varieties, or in fall for short day types. Sow in rows ¼” deep and 15” apart. They should be gradually thinned until they are about 4”-6” apart - the thinnings can be used as green onions or may be transplanted.
For intermediate day onions I have a much greater success sowing indoors in cell packs in mid-January. I harden them off (outside in a protected place during the day, but in at night) ready to plant them out as tiny plants once they have two true leaves which is about one month after sowing. I put one seed per cell so that there is little disturbance to the plants when I plant them out.
I have also used cardboard fruit boxes such as tangerines come in, filled with seed compost and placed on a tray. The box usually lasts for about one month by which time the cardboard is on the point of disintegrating which makes removing the little transplants easy.
Commercially grown transplants
These are an easy way to grow onions if seed-starting is not possible. Many catalogs now carry Intermediate day onions as small plants, and even sell them in mixed bunches of red, yellow and white varieties as a convenience. They have their roots and tops trimmed, and depending on the company may have been lifted some time before they arrive so they may be rather dried out. Also some companies don't send them out very early in the season to zone 7 – a point worth checking.
They are comparatively expensive and come in large bundles of about 50-70 plants which is generally more than I need so it may help to find someone willing to share an order.
These are tiny onion bulbs which are grown the previous season in crowded conditions to keep them under-developed and are then lifted and dried. They appear in net bags and are usually just labeled “white” or “yellow” with no varietal name and are most often long-day types. My experience with trying to grow these is that they bolt very readily or just grow foliage and some very small bulbs. In this area they are usually planted in the fall, but are often already woody in the middle before they are lifted. They may be planted to provide a supply of green onions, and can be planted close together for this purpose.
My experience has been with using drip irrigation with ½ pipes and emitters every 1 foot and this I bury about just under the surface. During the recent drought years I have planted a row on either side of the pipe with those plants on one side about 2” to the left of an emitter, and the ones on the other side about 2” to the right so that they were checkerboarded and the plants were about 4”-5”away from each other. If growing with soaker hoses or overhead irrigation they can be planted about 4” apart depending on the cultivar. Closer spacing will give smaller onions.
One month after planting I mulch the growing plants to conserve moisture and prevent sunscald of the developing bulbs, and then just leave them to grow.
I have good, regularly amended soil so I fertilize only a couple of times using a general liquid fertilizer early in the growing season (March and May). If growing sweet onions avoid using fertilizers containing sulfur. It is important not to fertilize once bulbing has begun. The areas between the plantings remained dry because of lack of water, and very few weeds appeared through the mulch but these were removed when small to avoid disturbing the onion roots.
As the foliage grows take care not to break or bend it at the neck as this will inhibit nutrition from the foliage reaching the bulb.
Harvesting and Curing.
When the bulbs have grown to full size, usually around early August for intermediate-day varieties, the foliage begins to turn brown and bend over at the necks. At that point I remove the mulch, reduce the irrigation, and as the onions dry I disconnect the irrigation pipe completely. When the foliage is dry I lift the onions very carefully so as not to damage them or tear off the drying skins. They have usually grown so that the bulb is almost sitting on top of the soil. Then the onions are traditionally laid out on the soil surface to dry, with the foliage of one plant laid over the next to prevent sunburn. However, I have found that since this is often at the hottest time of year in our area, they tend to get sun damage anyway, so I have found that using an old mesh window screen supported on a couple of saw horses and placed away from strong, direct sunlight often gives better results.
Once the necks are completely dry, and the outer skins papery, the onions may be placed in mesh bags, or if suitable braided and stored in a cool, airy place. (Unfortunately, almost an impossibility here in the height of summer). I use a spare bedroom for the hotter months before transferring to the garage once the outside temperature cools off. Inspect for softening or rotting and remove those that are affected immediately; use any with thick or soft necks first.
Pests and Diseases.
I grew onions without any noticeable problems for a few years, but two years ago I began to get thrips infestations which affect onions grown in dry conditions, and appeared in July. The foliage became streaky and slightly grayish and sticky. Insecticidal soap has proved effective in controlling this pest, which is not so likely to occur where onions have sprinkler irrigation. Damage to the crop was not severe since by that time the bulbs were well formed.
Onion pink root is a fungal disease and can be devastating. So far I have never seen any evidence of it in my crops.
For other pests and disease symptoms check the California Master Gardener Handbook.
Bolting, or sending up a premature flower spike during the first year of growth, is often the result of a sudden change in temperature, irregular irrigation or other stress. Some varieties are more prone to bolting than others, so check for this when choosing varieties. If a flower spike emerges, snip off the top of the stem, and use that onion quickly because it will not store. The center of the bulb may be woody, but the rest is still usable.
Yields vary but in 2016 I grew one 17' row of Candy from seed started early indoors and planted out as tiny plants in mid-February - a total of 35 plants which yielded just under 31lbs of onions, the largest being 1lb 6¾oz, and the smallest 9 oz. They stored through mid-December.
However another Master Gardener in a nearby neighborhood grew Candy from commercial transplants planted out on March 29 as part of a comprehensive trial of onions in 2014. She grew 30 plants of Candy yet the yield of those was only 11lbs 9oz, and they grew to only 2.5” in diameter.
//ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=16873 (Bishop trial of 9 different varieties)
//ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=19933 Fall planted onions in Bishop